1. Field of the Invention
The invention relates to a display apparatus comprising a display memory, means for storing in the display memory lines of image data read from a removable storage device, and a display controller constructed for reading in sequence the lines of image data stored in the memory to enable the synchronous display of a raster scan image comprising lines of pixel values defined by the stored lines of image data. The invention further relates to a method of storing an image for subsequent display by such an apparatus and to a removable storage device wherein an image has been stored by such a method.
2. Description of the Related Art
Such display apparatus are known generally in the art, but a particular example is the Compact Disc Interactive (CD-I) player described for example in "CD-I--a Designer's Overview" published by Kluwer (ISBN 9020121103).
In the CD-I system, various image coding options are available. For example, natural photographic images can be encoded and recorded on an optical disc for subsequent retrieval, decoding and display by the CD-I player. The encoding technique used achieves a high degree of data compression, but the data rate of the data channel (the disc reading apparatus) is not high enough to permit sequences of such images to be displayed as moving pictures unless the images are limited to a small area of the available display screen.
Such images can be expanded in the known apparatus to fill a larger screen area, but only by repeating the received pixel values to display each value over a large block of pixels. This technique introduces a "mosaic" effect, however, which is visually obtrusive and generally undesirable.
It is known that satisfactory expansion of digitised images can be achieved by using linear interpolation to generate intermediate pixel values. The interpolation can be performed substantially independently in the line scan (hereinafter "horizontal") and field scan (hereinafter "vertical") directions.
Techniques for performing vertical interpolation are known, for example in television studio equipment such as that described in United Kingdom patent application GB-A-2 073 988. In known systems, special memory addressing hardware enables parallel access to two lines of stored pixel values and uses special arithmetic circuits to combine these to form the interpolated values. Such a technique is too expensive for use in high-volume consumer applications. Furthermore, in CD-I for example, images are stored in the differentially coded form in which they are received from the disc, and are only decoded to give the actual pixel values after being read synchronously with the raster scan display.
An alternative technique would be to provide a line buffer to store the decoded pixel values of a line so that they may be combined with values decoded for subsequent line. This solution is similar to that applied in a television display system described in U.S. Pat. No. 4,440,719, but it too is expensive for high-volume consumer applications.